Inner focusing lens

ABSTRACT

An inner focusing lens has sequentially from the object side, a first lens group having a positive refractive power, a second lens group having a negative refractive power, and a third lens group having a negative refractive power. The first lens group has negative meniscus lenses disposed farthest on the object side thereof. The second lens group is moved along the optical axis whereby focusing from a focus state for an object at infinity to a focus state for the minimum object distance is performed. The inner focusing lens satisfies predetermined conditions and thereby, realizes a compact inner focusing lens having high imaging performance at wide angles, suitable for compact cameras having a function of capturing video.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is based upon and claims the benefit of priority of theprior Japanese Patent Application No. 2015-069443, filed on Mar. 30,2015, the entire contents of which are incorporated herein by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a compact inner focusing lens havinghigh imaging performance.

2. Description of the Related Art

Conventionally, a relatively long flange focal length has to beestablished with respect to the focal length and thus, many lenses forsingle-lens reflex cameras adopt a configuration that includes apositive lens group toward the rear of the optical system to easilyestablish back focus. Nonetheless, in recent years, camera bodies havedecreased in size and consequent to the spread of digital cameras,instances where long flange focal lengths are not necessary areincreasing.

Further, since video can also be captured by a digital camera,high-speed autofocus processing is desirable. In autofocus processing, aportion of a lens group (focusing group) is moved rapidly along theoptical axis (wobble) to achieve transitions from a non-focused state→afocused state→a non-focused state. Further, a signal component of aspecific frequency band of a partial image area is detected from theoutput signal of the image sensor; an optimal position of the focusinggroup achieving a focused state is determined; and the focusing group ismoved to the optimal position. In particular, when video is captured,this series of operations has to be rapidly continued, repeatedly.Further, in the execution of wobble, the focusing group has to belight-weight and have the smallest diameter possible to enable rapiddriving of the focusing group.

When a positive lens group is disposed farthest on the image side of anoptical system, the refractive power of the focusing group has to besomewhat strong and therefore, the optical system becomes susceptible toaberration variations consequent to wobble and the effects ofmagnification. To suppress aberration variations consequent to wobbleand the effects of magnification when focusing is performed, disposal ofa negative element farthest on the image side of the optical system isdesirable.

Thus, to address such demands, an inner focusing lens that can alsosufficiently cope with video filming has been proposed (for example,refer to Japanese Patent Application Laid-Open Publication No.2013-97212).

The inner focusing lens disclosed in Japanese Patent ApplicationLaid-Open Publication No. 2013-97212 has sequentially from an objectside, a first lens group having a positive refractive power, a secondlens group having a negative refractive power, and a third lens grouphaving a negative refractive power, where the second lens group is movedto perform focusing. The inner focusing lens has a medium telephotofocal length by a 35 mm film camera conversion and a small, light-weightfocusing group internally.

On the other hand, conventionally, at an image sensor that opticallyreceives and converts an optical image into an electronic image signal,there are limitations on the efficiency of taking in incident light bythe on-chip micro lens, etc. and on the lens side, it is desirable forthe exit pupil to be made to be greater than a certain diameter and toassure telecentricity of the luminous flux incident to the image sensor.

Nonetheless, with recent image sensors, improved aperture ratios andadvances in the design freedom of on-chip micro lenses have reduced exitpupil limitations demanded on the imaging lens side. Furthermore, withrecent software and camera system advances and improvements, even whendistortion is significant to an extent that conventionally, thedistortion would be conspicuous, correction by image processing hasbecome possible.

Therefore, in conventional image lenses, although a positive lenselement is disposed farthest on the image side of the optical system andtelecentricity is assured, in recent years, this is no longer necessaryand even when a negative lens element is disposed farthest on the imageside of the optical system and there is oblique incidence of theluminous flux on the image sensor, vignetting (shading) consequent tomismatching of the on-chip micro lens and pupil, etc. has becomeinconspicuous. Further, since a negative lens element can now bedisposed farthest on the image side of an optical system, reductions inthe diameter of optical systems can be expected.

In contrast, with the inner focusing lens disclosed in Japanese PatentApplication Laid-Open Publication No. 2013-9722, since a positive lenselement is disposed farthest on the image side of the optical system,which has a shorter overall length, the diameter of the third lens group(lens farthest on the image side) cannot be sufficiently reduced.Therefore, application to cameras having a smaller dimension along thedirection of diameter of the optical system is difficult, such asmirrorless interchangeable-lens cameras that have come into wide use.

The inner focusing lens disclosed in Japanese Patent ApplicationLaid-Open Publication No. 2013-97212 is not aimed for wide angle viewsand therefore, the correction of field curvature and distortion as wellas assuring the amount of light at the lens ends necessary for wideangle views are points that have not been considered.

SUMMARY OF THE INVENTION

According to one aspect of the present invention an inner focusing lensincludes sequentially from an object side thereof, a first lens grouphaving a positive refractive power; a second lens group having anegative refractive power; and a third lens group having a negativerefractive power. The first lens group includes farthest on an objectside thereof, at least one negative meniscus lens. Focusing is performedfrom a focus state at infinity to a focus state for a minimum objectdistance by moving the second lens group along an optical axis, from theobject side to an image side such that an interval between the firstlens group and the second lens group increases and an interval betweenthe second lens group and the third lens group decreases, while thefirst lens group and the third lens group remain fixed. The innerfocusing lens satisfies a conditional expression (1) f3/f≦−29.1, wheref3 is a focal length of the third lens group at the focus state atinfinity and f is an overall optical-system focal length at the focusstate at infinity.

The other objects, features, and advantages of the present invention arespecifically set forth in or will become apparent from the followingdetailed description of the invention when read in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a diagram depicting, along the optical axis, a configurationof an inner focusing lens according to a first embodiment;

FIG. 2 is a diagram of various types of aberration occurring in theinner focusing lens according to the first embodiment;

FIG. 3 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a second embodiment;

FIG. 4 is a diagram of various types of aberration occurring in theinner focusing lens according to the second embodiment;

FIG. 5 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a third embodiment;

FIG. 6 is a diagram of various types of aberration occurring in theinner focusing lens according to the third embodiment;

FIG. 7 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a fourth embodiment;

FIG. 8 is a diagram of various types of aberration occurring in theinner focusing lens according to the fourth embodiment.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Preferred embodiments of an inner focusing lens according to the presentinvention will be described in detail.

The inner focusing lens according to the present invention is configuredby a first lens group having a positive refractive power, a second lensgroup having a negative refractive power, and a third lens group havinga negative refractive power, sequentially disposed from an object side.

In the inner focusing lens according to the present invention, the firstlens group and the third lens group are fixed, while the second lensgroup is moved along an optical axis from an object side to an imageside such that an interval between the first lens group and the secondlens group increases and an interval between the second lens group andthe third lens group decreases, whereby focusing from a focus state foran object at infinity to a focus state for the minimum object distanceis performed. In this manner, by moving the second lens group to performfocusing, protection against dust and sound-proofing performance areenhanced without changes in the overall length of the optical system.

Further, by disposing farthest on the object side, the first lens grouphaving a positive refractive power, the diameter of the luminous fluxguided to the subsequent second lens group can be reduced. Therefore,the diameter of the second lens group, which is the focusing group, isreduced, enabling a reduction in the weight of second lens group to befacilitated. As a result, high-speed, silent focusing becomes possible,which is beneficial in capturing video. Further, since the diameter ofthe second lens group can be reduced, this is advantageous in reducingthe diameter of the optical system.

Further, by disposing at least one negative meniscus lens farthest onthe object side of the first lens group, wide angle views by the opticalsystem are facilitated.

By disposing farthest on the image side, the third lens group having anegative refractive power, a default ratio (total length/focal length)is reduced, enabling a reduction of the back focus and furtherreductions in the size of the optical system.

To provide a compact inner focusing lens having a compact, light-weightfocusing group, a wide angle focal length, and high imaging performance,in addition to the configuration described above, various conditionssuch as the following are set concerning the present invention.

In the inner focusing lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, f3 isthe focal length of the third lens group at a focus state for an objectat infinity; and f is the focal length of the optical system overall ata focus state for an object at infinity.

f3/f≦−29.1  (1)

Conditional expression (1) prescribes a ratio of the focal lengths ofthe third lens group and the entire optical system at the focus statefor an object at infinity. By satisfying conditional expression (1), therefractive power of the third lens group is optimized, enablingreductions in the diameter and overall length of the optical systemwhile maintaining high imaging performance.

Above the upper limit of conditional expression (1), the refractivepower of the third lens group becomes strong. In this case, the F numberin the optical system overall tends to be large and a bright opticalsystem cannot be obtained. To realize a bright optical system in thisstate, the aperture stop has to be opened widely. However, if theaperture stop is opened widely, various types of aberration becomeprominent and therefore, to realize an optical system having favorableimaging performance, the number of lenses for correcting aberration hasto be increased. In particular, the number of lenses configuring thefirst lens group has to be increased. When the optical system isconfigured by a large number of lenses, reductions in the size andweight of the optical system become difficult and thus, a large numberof lenses is not desirable.

By satisfying conditional expression (1) within the following range, amore favorable effect can be expected.

−1200≦f3/f≦−30.1  (1a)

By satisfying conditional expression (1a) within the following range, aneven more favorable effect can be expected.

−10000≦f3/f≦−31.1  (1b)

In the inner focusing lens according to the present invention, thefollowing conditional expression is preferably satisfied; where f1 isthe focal length of the first lens group at a focus state for an objectat infinity and f is the focal length of the optical system overall at afocus state for an object at infinity.

0.28≦f1/f≦1.30  (2)

Conditional expression (2) prescribes a ratio of the focal lengths ofthe first lens group and the optical system overall, at a focus statefor an object at infinity. By satisfying conditional expression (2), thefocal length of the first lens group becomes suitable with respect tothe focal length of the optical system overall, enabling reductions inthe overall length of the optical system and in the diameters ofsubsequent lenses, and improved imaging performance.

Below the lower limit of conditional expression (2), the focal length ofthe first lens group becomes short and spherical aberration correctionbecomes insufficient or paraxial magnification of a subsequent lensgroup increases and the diameters of subsequent lenses increase,increasing the size of the optical system, and thus, is not desirable.On the other hand, above the upper limit of conditional expression (2),the focal length of the first lens group increases, increasing theoverall length of the optical system, making size reductions of theoptical system difficult.

By satisfying conditional expression (2) within the following range, amore favorable result can be expected.

0.37≦f1/f≦0.97  (2a)

By satisfying conditional expression (2a) within the following range, aneven more favorable result can be expected.

0.45≦f1/f≦0.80  (2b)

In the inner focusing lens according to the present invention, thefollowing conditional expression is preferably satisfied; where βinf isthe paraxial magnification of the second lens group at a focus state foran object at infinity and β mod is the paraxial magnification of thesecond lens group at a focus state for the minimum object distance.

0.50≦βinf/β mod≦2.02  (3)

Conditional expression (3) prescribes a ratio of the paraxial transversemagnification of the second lens group at a focus state for an object atinfinity and at a focus state for the minimum object distance. Bysatisfying conditional expression (3), changes in magnification can besuppressed even when the focusing group (second lens group) is moved andfield of view variations can be suppressed during focusing. If the rangeprescribed by conditional expression (3) is deviated from, field of viewvariations cannot be suppressed during focusing. If field of viewvariation occurs while the focusing group is moving, the image looksblurred and image quality drops.

By satisfying conditional expression (3) within the following range, amore favorable effect can be expected.

0.60≦βinf/β mod≦1.80  (3a)

By satisfying conditional expression (3a) within the following range, aneven more favorable effect can be expected.

0.68≦βinf/β mod≦1.60  (3b)

By further satisfying conditional expression (3b) within the followingrange, field of view variations during focusing can be made extremelysmall.

0.80≦βinf/β mod≦1.40  (3c)

In the inner focusing lens according to the present invention, the thirdlens group includes sequentially from the object side, a front lenssubgroup having a positive refractive power and a rear lens subgrouphaving a negative refractive power; and an axial air gap that is widestin the third lens group is formed between the front lens subgroup andthe rear lens subgroup.

By such a configuration, the lens diameter near the imaging plane isreduced and imaging performance can be improved. In other words,increased lens diameter on the image side (an issue in reducing the sizeof optical systems having a short flange focal length and for mountingon compact cameras such as mirrorless interchangeable-lens cameras) canbe suppressed by disposing on the object side of the third lens group,the front lens subgroup having a positive refractive power. Further, bydisposing the rear lens subgroup (having a negative refractive power) onthe image side of the front lens subgroup to form an air gap, axialaberration can be corrected by the front lens subgroup having a positiverefractive power; and at the rear lens subgroup, off axis aberration, inparticular, distortion, can be favorably corrected.

In the inner focusing lens according to the present invention, a simplelens element having a negative refractive power is preferably disposedfarthest on the image side of the third lens group. With suchconfiguration, the diameter of the third lens group (lens farthest onthe image side) can be further reduced, which is optimal for compactcameras such as mirrorless interchangeable-lens cameras that have comeinto wide-spread use in recent years.

A simple lens element includes a single ground lens, an aspheric lens, acompound aspheric lens, and a cemented lens; and a simple lens elementdoes not include, for example, two positive lenses, etc. where theelements are not bonded to one another to have a layer of airtherebetween.

In addition to disposing farthest on the image side of the third lensgroup, a simple lens element having a negative refractive power, in theinner focusing lens according to the present invention, the followingconditional expression is preferably satisfied, where R1 is the radiusof curvature of the surface at an air interface, on the object side ofthe simple lens element having a negative refractive power; and R2 isthe radius of curvature of the surface at an air interface, on the imageside of the simple lens element having a negative refractive power.

0≦(R1+R2)/(R1−R2)  (4)

Conditional expression (4) prescribes the shape of the simple lenselement having a negative refractive power and disposed farthest on theimage side of the third lens group. By satisfying conditional expression(4), the radius of curvature of the surface on the object side of thesimple lens element becomes greater than the radius of curvature of thesurface on the image side thereof. As a result, favorable correction ofoff-axis coma becomes possible.

By satisfying conditional expression (4) within the following range, amore favorable effect can be expected.

0.5≦(R1+R2)/(R1−R2)  (4a)

By satisfying conditional expression (4a) within the following range,more effective correction of off axis coma is achieved.

0.7≦(R1+R2)/(R1−R2)≦100  (4b)

The inner focusing lens according to present invention has an aperturestop that sets a predetermined diameter and in the inner focusing lensthe following conditional expression is preferably satisfied, where L1sis the axial distance from the surface farthest on the object side ofthe first lens group to the aperture stop, and L is the axial distance(overall length of the optical system) from the apex of the lens surfacefarthest on the object side in the optical system to the imaging plane.

0.24≦L1s/L≦0.95  (5)

Conditional expression (5) prescribes a ratio of the axial distance fromthe surface farthest on the object side of the first lens group to theaperture stop and the axial distance of the entire optical system. Bysatisfying conditional expression (5), an optimal position of theaperture stop is determined with respect to the overall length of theoptical system, enabling reduction of the optical system diameter whilemaintaining high imaging performance.

Below the lower limit of conditional expression (5), the aperture stopis too close to the object side, the lens diameter on the image sideincreases, and at the rear group, the occurrence of off-axis aberration,primarily distortion, becomes conspicuous and therefore, is notdesirable. On the other hand, above the upper limit of conditionalexpression (5), the aperture stop is to too close to the image side andwith the increase in the effective diameter of the front lens, sizereductions of the optical system become difficult. The aperture stop ispreferably disposed in the first lens group.

By satisfying conditional expression (5) within the following range, amore favorable effect can be expected.

0.32≦L1s/L≦0.71  (5a)

By satisfying conditional expression (5a) within the following range, aneven more favorable effect can be expected.

0.40≦L1s/L≦0.60  (5b)

In the inner focusing lens according to the present invention,preferably, the second lens group is configured by a simple lens elementhaving a negative refractive power. The definition of the simple lenselement here is as described above.

By configuring the second lens group by a simple lens element having anegative refractive power, reductions in the size and weight of thefocusing group can be achieved, enabling faster focusing, which isbeneficial in capturing video. Reductions in the size and weight of thefocusing group decrease the load on the driving mechanism such as anactuator for driving the focusing group, contributing to reduced powerconsumption. Further, the capacity of the driving mechanism can bereduced.

In the inner focusing lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, f2 isthe focal length of the second lens group at a focus state for an objectat infinity; and f is the focal length of the optical system at a focusstate for an object at infinity.

−7.46≦f2/f≦−2.11  (6)

Conditional expression (6) prescribes a ratio of the focal length of thesecond lens group and the focal length of the optical system overall, ata focus state for an object at infinity. By satisfying conditionalexpression (6), reductions in the size of the optical system can berealized and high imaging performance can be maintained (particularlyeffective in correcting field curvature).

Below the lower limit of conditional expression (6), the focal length ofthe second lens group increases and the negative power of the secondlens group becomes too weak. As a result, the distance the second groupis moved during focusing increases, whereby the overall length of theoptical system increases, making reductions in the size of the opticalsystem difficult. On the other hand, above the upper limit ofconditional expression (6), the focal length of the second lens groupdecreases and the negative power of the second lens group becomes toostrong. As a result, field of view variation and aberration variation(particularly, variation of field curvature) accompanying movement ofthe second lens group during focusing become excessive, which is notdesirable.

By satisfying conditional expression (6) within the following range, amore favorable effect can be expected.

−5.60≦f2/f≦−1.80  (6a)

By satisfying conditional expression (6a) within the following range, aneven more favorable effect can be expected.

−5.00≦f2/f≦−1.62  (6b)

In the inner focusing lens according to the present invention,preferably, the third lens group is configured to include a simple lenselement having a negative refractive power and more preferably, thesimple lens element is formed of a single glass material. By forming thesimple lens element in the third lens group of a single glass material,a reduction in a thickness and diameter of the simple lens element canbe facilitated. Further, a reduction of the weight of the simple lenselement can be facilitated.

The inner focusing lens according to the present invention preferablysatisfies the following conditional expression where win is the Abbenumber with respect to d-line of the simple lens having a negativerefractive power.

30≦νdn  (7)

Below the lower limit of conditional expression (7), chromaticdifference of magnification is overcorrected, making high imagingperformance difficult to maintain and therefore, is undesirable.

In the inner focusing lens according to the present invention, thefollowing conditional expression is preferably satisfied; where, R21 isthe radius of curvature of the surface farthest on the image side of thesecond lens group and R22 is the radius of curvature of the surfacefarthest on the image side of the second lens group.

0≦(R21+R22)/(R21−R22)  (8)

Conditional expression (8) prescribes the shapes of the surfacesfarthest on the object side and on the image side of the second lensgroup. By satisfying conditional expression (8), in the second lensgroup, the radius of curvature of the surface farthest on the image sidebecomes smaller than the radius of curvature of the surface farthest onthe object side. As a result, variation of the angle of the light raysincident on the surface and having a strong power becomes small,enabling variation of the field curvature during focusing to besuppressed.

By satisfying conditional expression (8) within the following range, amore favorable result can be expected.

1≦(R21+R22)/(R21−R22)  (8a)

By satisfying conditional expression (8) within the following range, aneven more favorable result can be expected.

1≦(R21+R22)/(R21−R22)≦300  (8b)

In the inner focusing lens according to the present invention, disposala positive aspheric lens in the first lens group is beneficial incorrecting spherical aberration. In particular, by forming the positivelens to have an aspheric surface that weakens the power of paraxialcurvature, the effectiveness of spherical aberration correction isimproved.

In the inner focusing lens according to the present invention, byforming an aspheric surface on a lens configuring the second lens group,correction of field curvature becomes more effective. In particular, byforming a lens configuring the second lens group to have an asphericsurface that weakens the power of paraxial curvature, correction offield curvature is further improved and the effect of suppressing fieldcurvature variation during focusing becomes higher.

In the inner focusing lens according to the present invention, formingan aspheric surface on a lens of the third lens group is beneficial incorrecting field curvature. In particular, by forming a lens of thethird lens group to have an aspheric surface that weakens the power ofparaxial curvature, the corrective effect on field curvature isimproved.

As described above, according to the present invention, a compact innerfocusing lens can be realized that has high imaging performance, a wideangle focal length, and a compact, light-weight focusing group. Inparticular, by satisfying the conditional expressions above, a compactinner focusing lens having high imaging performance and suitable forcapturing video can be realized.

Embodiments of the inner focusing lens according to the presentinvention will be described in detail with reference to the accompanyingdrawings. The invention is not limited by the embodiments below.

FIG. 1 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a first embodiment. FIG. 1depicts a focus state for an object at infinity. The inner focusing lensincludes sequentially from an object side nearest a non-depicted object,a first lens group G₁₁ having a positive refractive power, a second lensgroup G₁₂ having a negative refractive power, and a third lens group G₁₃having a negative refractive power. A cover glass CG is disposed betweenthe third lens group G₁₃ and the imaging plane IMG.

The first lens group G₁₁ has sequentially from the object side, anegative meniscus lens L₁₁₁, a negative meniscus lens L₁₁₂, a negativelens L₁₁₃, a positive lens L₁₁₄, a negative lens L₁₁₅, a positive lensL₁₁₅, an aperture stop STP setting a predetermined diameter, and apositive lens L₁₁. Both surfaces of the negative lens L₁₁₃ are aspheric.The negative lens L₁₁, and the positive lens L₁₁₆ are cemented. Bothsurfaces of the positive lens L₁₁₇ are aspheric.

The second lens group G₁₂ is configured by a negative lens L₁₂₁. Bothsurfaces of the negative lens L₁₂₁ are aspheric.

The third lens group G₁₃ has sequentially from the object side, a frontlens subgroup G_(13F) having a positive refractive power and a rear lenssubgroup G_(13R) having a negative refractive power. An axial air gapthat is widest in the third lens group G₁₃ is formed between the frontlens subgroup G_(13F) and the rear lens subgroup G_(13R).

The front lens subgroup G_(13F) has sequentially from the object side, apositive lens L₁₃₁ and a positive lens L₁₃₂. The rear lens subgroupG_(13R) is configured by a negative lens L₁₃. The negative lens L₁₃₃ isformed of a single glass material. Both surfaces of the negative lensL₁₃₃ are aspheric.

In the inner focusing lens, the first lens group G₁₁ and the third lensgroup G₁₃ are fixed, while the second lens group G₁₂ is moved along theoptical axis from the object side to an imaging plane IMG side such thatthe interval between the first lens group G₁₁ and the second lens groupG₁₂ is increased and the interval between the second lens group G₁₂ andthe third lens group G₁₃ is decreased, whereby focusing is performedfrom a focus state for an object at infinity to a focus state for theminimum object distance.

Here, various values related to the inner focusing lens according to thefirst embodiment are given.

(Lens Data) r₁ = 28.832 d₁ = 2.000 nd₁ = 1.5935 νd₁ = 67.00 r₂ = 15.623d₂ = 6.886 r₃ = 29.397 d₃ = 1.500 nd₂ = 1.4970 νd₂ = 81.61 r₄ = 13.122d₄ = 5.883 r₅ = 52.568 (aspheric surface) d₅ = 1.300 nd₃ = 1.5920 νd₃ =67.02 r₆ = 12.910 (aspheric surface) d₆ = 3.068 r₇ = 30.333 d₇ = 2.265nd₄ = 1.8810 νd₄ = 40.14 r₈ = 141.737 d₈ = 12.099 r₉ = 24.676 d₉ = 1.000nd₅ = 1.8810 νd₅ = 40.14 r₁₀ = 13.601 d₁₀ = 5.292 nd₆ = 1.4875 νd₆ =70.44 r₁₁ = −35.502 d₁₁ = 1.300 r₁₂ = ∞ (aperture stop) d₁₂ = 2.425 r₁₃= 30.850 (aspheric surface) d₁₃ = 4.386 nd₇ = 1.4971 νd₇ = 81.56 r₁₄ =−18.338 (aspheric surface) d₁₄ = D(14) (variable) r₁₅ = 45.781 (asphericsurface) d₁₅ = 0.800 nd₈ = 1.7290 νd₈ = 54.04 r₁₆ = 19.193 (asphericsurface) d₁₆ = D(16) (variable) r₁₇ = −68.737 d₁₇ = 2.535 nd₉ = 1.4970νd₉ = 81.61 r₁₈ = −25.555 d₁₈ = 0.100 r₁₉ = −329.577 d₁₉ = 4.036 nd₁₀ =1.4970 νd₁₀ = 81.61 r₂₀ = −23.042 d₂₀ = 0.329 r₂₁ = −400.000 (asphericsurface) d₂₁ = 1.200 nd₁₁ = 1.8820 νd₁₁ = 37.22 r₂₂ = 26.785 (asphericsurface) d₂₂ = 25.606 r₂₃ = ∞ d₂₃ = 2.500 nd₁₂ = 1.5168 νd₁₂ = 64.20 r₂₄= ∞ d₂₄ = 1.000 r₂₅ = ∞ (imaging plane) Constant of the Cone (k) andAspheric Coefficients (A₄, A₆, A₈, A₁₀) (Fifth Order) k = 0, A₄ =−2.6625 × 10⁻⁵, A₆ = 3.0031 × 10⁻⁷, A₈ = −1.7989 × 10⁻⁹, A₁₀ = 5.7847 ×10⁻¹² (Sixth Order) k = 0, A₄ = −6.8334 × 10⁻⁵, A₆ = 8.4364 × 10⁻⁹, A₈ =−1.2228 × 10⁻⁹, A₁₀ = −9.8374 × 10⁻¹² (Thirteenth Order) k = 0, A₄ =−2.2269 × 10⁻⁵, A₆ = 1.1253 × 10⁻⁹, A₈ = −1.0562 × 10⁻⁹, A₁₀ = −3.1969 ×10⁻¹² (Fourteenth Order) k = 0, A₄ = 5.1791 × 10⁻⁵, A₆ = −5.2286 × 10⁻⁷,A₈ = 4.0083 × 10⁻⁹, A₁₀ = 2.3692 × 10⁻¹¹ (Fifteenth Order) k = 0, A₄ =2.0348 × 10⁻⁵, A₆ = −1.1141 × 10⁻⁶, A₈ = 1.4175 × 10⁻⁸, A₁₀ = −5.7786 ×10⁻¹¹ (Sixteenth Order) k = 0, A₄ = 2.1580 × 10⁻⁵, A₆ = −8.9505 × 10⁻⁷,A₈ = 1.2780 × 10⁻⁸, A₁₀ = −5.7413 × 10⁻¹¹ (Twenty-first Order) k = 0, A₄= −2.4151 × 10⁻⁵, A₆ = −1.3394 × 10⁻⁷, A₈ = 2.2182 × 10⁻⁹, A₁₀ = −7.5852× 10⁻¹² (Twenty-second Order) k = 0, A₄ = −3.5206 × 10⁻⁶, A₆ = −1.2925 ×10⁻⁷, A₈ = 2.2655 × 10⁻⁹, A₁₀ = −8.5817 × 10⁻¹² (Focal State Data)Minimum Object Distance Infinity (object distance 158.000 mm) D(14)1.472 2.318 D(16) 6.519 5.674 f (focal length of optical 18.54 17.88system overall) FNO (F number) 2.88 2.92 ω (half angle of view) 50.2949.76 f1 (focal length of first 10.28 10.28 lens group G₁₁) f2 (focallength of second −45.92 −45.92 lens group G₁₂) f3 (focal length of third−722.75 −722.75 lens group G₁₃) BF (back focus) 29.106 29.106

(Values Related to Conditional Expression (1))

f3/f=−38.97

(Values Related to Conditional Expression (2))

f1/f=0.55

(Values Related to Conditional Expression (3))

βinf (paraxial magnification of second lens group G₁₂ at focus state forobject at infinity)=1.83β mod (paraxial magnification of second lens group G₁₂ at focus statefor minimum object distance)=1.81βinf/β mod=1.01

(Values Related to Conditional Expression (4))

R1 (radius of curvature of surface at air interface on object side ofnegative lens L₁₃₃)=−400.000R2 (radius of curvature of surface at air interface on image side ofnegative lens L₁₃)=26.785

(R1+R2)/(R1−R2)=0.87 (Values Related to Conditional Expression (5))

L1s (axial distance from surface farthest on object side of first lensgroup G₁₁ to aperture stop STP)=42.593L (axial distance from apex of lens surface farthest on object side offirst lens group G₁₁ to imaging plane IMG (overall length of opticalsystem))=95.501

L1s/L=0.45 (Values Related to Conditional Expression (6))

f2/f=−2.48

(Values Related to Conditional Expression (7))

νdn (Abbe number with respect to d-line of negative lens L₁₃₃)=37.22

(Values Related to Conditional Expression (8))

R21 (radius of curvature of surface farthest on object side of negativelens L₁₂₁)=45.781R22 (radius of curvature of surface farthest on image side of negativelens L₁₂₁)=19.193

(R21+R22)/(R21−R22)=2.44

FIG. 2 is a diagram of various types of aberration occurring in theinner focusing lens according to the first embodiment. In the diagram,for curves depicting spherical aberration, the vertical axis representsthe F number (Fno), solid lines depict wavelength characteristicscorresponding to d-line (λ=587.56 nm), dotted lines depict wavelengthcharacteristics corresponding to g-line (λ=435.84 nm), and dashed linesdepict wavelength characteristics corresponding to C-line (λ=656.28 nm).For curves depicting astigmatism, the vertical axis represents themaximum image height (Y), S represents characteristics of the sagittalplane and M represents characteristics of the meridional plane. Forcurves depicting distortion, the vertical axis represents the maximumimage height (Y) and wavelength characteristics corresponding to d-lineare depicted.

FIG. 3 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a second embodiment. FIG. 3depicts a focus state for an object at infinity. The inner focusing lenshas sequentially from the object side, a first lens group G₂₁ having apositive refractive power, a second lens group G₂₂ having a negativerefractive power, and a third lens group G₂₃ having a negativerefractive power. The cover glass CG is disposed between the third lensgroup G₂₃ and the imaging plane IMG.

The first lens group G₂₁ has sequentially from the object side, anegative meniscus lens L₂₁₁, a negative meniscus lens L₂₁₂, a negativelens L₂₁₃, a positive lens L₂₁₄, a negative lens L₂₁₅, a positive lensL₂₁₆, the aperture stop STP setting a predetermined diameter, and apositive lens L₂₁₇. Both surfaces of the negative lens L₂₁₃ areaspheric. The negative lens L₂₁₅ and the positive lens L₂₁₆ arecemented. Both surfaces of the positive lens L₂₁₇ are aspheric.

The second lens group G₂₂ is configured by a negative lens L₂₂₁. Bothsurfaces of the negative lens L₂₂₁ as aspheric.

The third lens group G₂₃ has sequentially from the object side, a frontlens subgroup G_(23F) having a positive refractive power and a rear lenssubgroup G_(23R) having a negative refractive power. An axial air gapthat is widest in the third lens group G₂₃ is formed between the frontlens subgroup G_(23F) and the rear lens subgroup G_(23R).

The front lens subgroup G_(23F) has sequentially from the object side, apositive lens L₂₃₁ and a positive lens L₂₃₂. The rear lens subgroupG_(23R) is configured by a negative lens L₂₃₃. The negative lens L₂₃₃ isformed of a single glass material. Both surfaces of the negative lensL₂₃₃ are aspheric.

In the inner focusing lens, the first lens group G₂₁ and the third lensgroup G₂₃ are fixed, while the second lens group G₂₂ is moved along theoptical axis from the object side to the imaging plane IMG side suchthat the interval between the first lens group G₂₁ and the second lensgroup G₂₂ is increased and the interval between the second lens groupG₂₂ and the third lens group G₂₃ is decreased, whereby focusing isperformed from a focus state for an object at infinity to a focus statefor the minimum object distance.

Here, various values related to the inner focusing lens according to thesecond embodiment are given.

(Lens Data) r₁ = 30.152 d₁ = 2.000 nd₁ = 1.5935 νd₁ = 67.00 r₂ = 16.214d₂ = 6.369 r₃ = 31.366 d₃ = 1.500 nd₂ = 1.4970 νd₂ = 81.61 r₄ = 13.499d₄ = 5.658 r₅ = 51.837 (aspheric surface) d₅ = 1.300 nd₃ = 1.5920 νd₃ =67.02 r₆ = 13.285 (aspheric surface) d₆ = 2.866 r₇ = 27.620 d₇ = 2.238nd₄ = 1.8810 νd₄ = 40.14 r₈ = 87.188 d₈ = 12.690 r₉ = 23.272 d₉ = 1.000nd₅ = 1.8810 νd₅ = 40.14 r₁₀ = 13.324 d₁₀ = 5.334 nd₆ = 1.4875 νd₆ =70.44 r₁₁ = −43.127 d₁₁ = 1.629 r₁₂ = ∞ (aperture stop) d₁₂ = 1.822 r₁₃= 30.203 (aspheric surface) d₁₃ = 4.424 nd₇ = 1.4971 νd₇ = 81.56 r₁₄ =−18.833 (aspheric surface) d₁₄ = D(14) (variable) r₁₅ = 42.496 (asphericsurface) d₁₅ = 0.800 nd₈ = 1.7290 νd₈ = 54.04 r₁₆ = 18.812 (asphericsurface) d₁₆ = D(16) (variable) r₁₇ = 647.460 d₁₇ = 3.783 nd₉ = 1.4970νd₉ = 81.61 r₁₈ = −19.801 d₁₈ = 0.100 r₁₉ = −48.734 d₁₉ = 2.492 nd₁₀ =1.4970 νd₁₀ = 81.61 r₂₀ = −33.384 d₂₀ = 0.761 r₂₁ = −400.000 (asphericsurface) d₂₁ = 1.200 nd₁₁ = 1.8820 νd₁₁ = 37.22 r₂₂ = 27.667 (asphericsurface) d₂₂ = 22.428 r₂₃ = ∞ d₂₃ = 2.500 nd₁₂ = 1.5168 νd₁₂ = 64.20 r₂₄= ∞ d₂₄ = 1.000 r₂₅ = ∞ (imaging plane) Constant of the Cone (k) andAspheric Coefficients (A₄, A₆, A₈, A₁₀) (Fifth Order) k = 0, A₄ =−2.7090 × 10⁻⁵, A₆ = 2.6283 × 10⁻⁷, A₈ = −1.4454 × 10⁻⁹, A₁₀ = 4.3803 ×10⁻¹² (Sixth Order) k = 0, A₄ = −6.1483 × 10⁻⁵, A₆ = 4.0703 × 10⁻⁸, A₈ =−1.1388 × 10⁻⁹, A₁₀ = −6.0150 × 10⁻¹² (Thirteenth Order) k = 0, A₄ =−2.2964 × 10⁻⁵, A₆ = −6.1034 × 10⁻⁹, A₈ = −1.0258 × 10⁻⁹, A₁₀ = 2.5147 ×10⁻¹² (Fourteenth Order) k = 0, A₄ = 4.6889 × 10⁻⁵, A₆ = −4.5934 × 10⁻⁷,A₈ = 3.4940 × 10⁻⁹, A₁₀ = −1.7504 × 10⁻¹¹ (Fifteenth Order) k = 0, A₄ =2.0032 × 10⁻⁵. A₆ = −1.0580 × 10⁻⁶, A₈ = 1.3913 × 10⁻⁸, A₁₀ = −6.6138 ×10⁻¹¹ (Sixteenth Order) k = 0, A₄ = 2.3359 × 10⁻⁵, A₆ = −8.3928 × 10⁻⁷,A₈ = 1.2186 × 10⁻⁸, A₁₀ = −6.4391 × 10⁻¹¹ (Twenty-first Order) k = 0, A₄= −2.3499 × 10⁻⁵, A₆ = −1.2412 × 10⁻⁷, A₈ = 2.2965 × 10⁻⁹, A₁₀ = −8.5091× 10⁻¹² (Twenty-second Order) k = 0, A₄ = −2.5375 × 10⁻⁶, A₆ = −1.1587 ×10⁻⁷, A₈ = 2.1605 × 10⁻⁹, A₁₀ = −8.6835 × 10⁻¹² (Focal State Data)Minimum Object Distance Infinity (object distance 158.000 mm) D(14)1.480 2.405 D(16) 6.626 5.702 f (focal length of optical 19.41 18.66system overall) FNO (F number) 2.88 2.92 ω (half angle of view) 49.1148.48 f1 (focal length of first 10.82 10.82 lens group G₂₁) f2 (focallength of second −46.97 −46.97 lens group G₂₂) f3 (focal length of third−626.90 −626.90 lens group G₂₃) BF (back focus) 25.928 25.928

(Values Related to Conditional Expression (1))

f3/f=−32.30

(Values Related to Conditional Expression (2))

f1/f=0.56

(Values Related to Conditional Expression (3))

βinf (paraxial magnification of second lens group G₂₂ at focus state forobject at infinity)=1.85β mod (paraxial magnification of second lens group G₂₂ at focus statefor minimum object distance)=1.83βinf/β mod=1.01

(Values Related to Conditional Expression (4))

R1 (radius of curvature of surface at air interface on object side ofnegative lens L₂₃₃)=−400.000R2 (radius of curvature of surface at air interface on image side ofnegative lens L₂₃₂)=27.667

(R1+R2)/(R1−R2)=0.87 (Values Related to Conditional Expression (5))

L1s (axial distance from surface farthest on object side of first lensgroup G₂₁ to aperture stop STP)=42.584L (axial distance from apex of lens surface farthest on object side offirst lens group G₂₁ to imaging plane IMG (overall length of opticalsystem))=92.000

L1s/L=0.46 (Values Related to Conditional Expression (6))

f2/f=−2.42

(Values Related to Conditional Expression (7))

νdn (Abbe number with respect to d-line of negative lens L₂₃₂)=37.22

(Values Related to Conditional Expression (8))

R21 (radius of curvature of surface farthest on object side of negativelens L₂₂₁)=42.496R22 (radius of curvature of surface farthest on image side of negativelens L₂₂₁)=18.812

(R21+R22)/(R21−R22)=2.59

FIG. 4 is a diagram of various types of aberration occurring in theinner focusing lens according to second embodiment. In the diagram, forcurves depicting spherical aberration, the vertical axis represents theF number (Fno), solid lines depict wavelength characteristicscorresponding to d-line (λ=587.56 nm), dotted lines depict wavelengthcharacteristics corresponding to g-line (λ=435.84 nm), and dashed linesdepict wavelength characteristics corresponding to C-line (λ=656.28 nm).For curves depicting astigmatism, the vertical axis represents themaximum image height (Y), S represents characteristics of the sagittalplane and M represents characteristics of the meridional plane. Forcurves depicting distortion, the vertical axis represents the maximumimage height (Y) and wavelength characteristics corresponding to d-lineare depicted.

FIG. 5 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a third embodiment. FIG. 5depicts a focus state for an object at infinity. The inner focusing lenshas sequentially from the object side, a first lens group G₃₁ having apositive refractive power, a second lens group G₃₂ having a negativerefractive power, and a third lens group G₃₃ having a negativerefractive power. The cover glass CG is disposed between the third lensgroup G₃₃ and the imaging plane IMG.

The first lens group G₃₁ has sequentially from the object side, anegative meniscus lens L₃₁₁, a negative meniscus lens L₃₁₂, a negativelens L₃₁₃, a positive lens L₃₁₄, a negative lens L₃₁₅, a positive lensL₃₁₆, the aperture stop STP setting a predetermined diameter, and apositive lens L₃₁₇. Both surfaces of the negative lens L₃₃₃ areaspheric. The negative lens L₃₁₅ and the positive lens L₃₁₆ arecemented. Both surfaces of the positive lens L₃₁₇ are aspheric.

The second lens group G₃₂ is configured by a negative lens L₃₂₁. Bothsurfaces of the negative lens L₃₂₁ are aspheric.

The third lens group G₃₃ has sequentially from the object side, a frontlens subgroup G_(33F) having a positive refractive power and a rear lenssubgroup G_(33F) having a negative refractive power. An axial air gapthat is widest in the third lens group G₃ is formed between the frontlens subgroup G_(33F) and the rear lens subgroup G_(33R).

The front lens subgroup G₃₃F has sequentially from the object side, apositive lens L₃₃₁, and a positive lens L₃₃₂. The rear lens subgroupG_(33F) is configured by a negative lens L₃₃₃. The negative lens L₃₃ isformed of a single glass material. Both surfaces of the negative lensL₃₃₃ are aspheric.

In the inner focusing lens, the first lens group G₁ and the third lensgroup G₃₃ are fixed, while the second lens group G₃₂ is moved along theoptical axis from the object side to the imaging plane IMG side suchthat the interval between the first lens group G₃₁ and the second lensgroup G₃₂ is increased and the interval between the second lens groupG₃₂ and the third lens group G₃₃ is decreased, whereby focusing isperformed from a focus state for an object at infinity to a focus statefor the minimum object distance.

Here, various values related to the inner focusing lens according to thethird embodiment are given.

(Lens Data) r₁ = 29.863 d₁ = 2.000 nd₁ = 1.5935 νd₁ = 67.00 r₂ = 15.214d₂ = 7.108 r₃ = 31.676 d₃ = 1.500 nd₂ = 1.4970 νd₂ = 81.61 r₄ = 12.821d₄ = 5.662 r₅ = 56.855 (aspheric surface) d₅ = 1.300 nd₃ = 1.5920 νd₃ =67.02 r₆ = 12.496 (aspheric surface) d₆ = 4.710 r₇ = 25.649 d₇ = 2.736nd₄ = 1.8810 νd₄ = 40.14 r₈ = −2872.181 d₈ = 10.427 r₉ = 29.340 d₉ =1.000 nd₅ = 1.8810 νd₅ = 40.14 r₁₀ = 11.062 d₁₀ = 5.282 nd₆ = 1.4875 νd₆= 70.44 r₁₁ = −36.758 d₁₁ = 1.300 r₁₂ = ∞ (aperture stop) d₁₂ = 3.832r₁₃ = 32.994 (aspheric surface) d₁₃ = 4.343 nd₇ = 1.4971 νd₇ = 81.56 r₁₄= −16.586 (aspheric surface) d₁₄ = D(14) (variable) r₁₅ = 168.747(aspheric surface) d₁₅ = 0.800 nd₈ = 1.7290 νd₈ = 54.04 r₁₆ = 35.344(aspheric surface) d₁₆ = D(16) (variable) r₁₇ = −47.172 d₁₇ = 2.755 nd₉= 1.4970 νd₉ = 81.61 r₁₈ = −20.329 d₁₈ = 0.100 r₁₉ = −45.060 d₁₉ = 3.412nd₁₀ = 1.4970 νd₁₀ = 81.61 r₂₀ = −20.831 d₂₀ = 0.833 r₂₁ = −400.000(aspheric surface) d₂₁ = 1.200 nd₁₁ = 1.8820 νd₁₁ = 37.22 r₂₂ = 34.153(aspheric surface) d₂₂ = 20.338 r₂₃ = ∞ d₂₃ = 2.500 nd₁₂ = 1.5168 νd₁₂ =64.20 r₂₄ = ∞ d₂₄ = 1.000 r₂₅ = ∞ (imaging plane) Constant of the Cone(k) and Aspheric Coefficients (A₄, A₆, A₈, A₁₀) (Fifth Order) k = 0, A₄= −2.3716 × 10⁻⁵, A₆ = 1.7317 × 10⁻⁷, A₈ = −6.4139 × 10⁻¹⁰, A₁₀ = 1.5464× 10⁻¹² (Sixth Order) k = 0, A₄ = −8.7141 × 10⁻⁵, A₆ = −3.1834 × 10⁻⁷,A₈ = 1.5577 × 10⁻⁹, A₁₀ = −2.7154 × 10⁻¹¹ (Thirteenth Order) k = 0, A₄ =−1.4834 × 10⁻⁵, A₆ = −1.6560 × 10⁻⁷, A₈ = 1.3132 × 10⁻⁹, A₁₀ = −1.6004 ×10⁻¹¹ (Fourteenth Order) k = 0, A₄ = 2.5383 × 10⁻⁵, A₆ = −2.6971 × 10⁻⁷,A₈ = 1.9137 × 10⁻⁹, A₁₀ = −1.9738 × 10⁻¹¹ (Fifteenth Order) k = 0, A₄ =4.8211 × 10⁻⁵, A₆ = −8.0172 × 10⁻⁷, A₈ = 1.0635 × 10⁻⁸, A₁₀ = −4.7053 ×10⁻¹¹ (Sixteenth Order) k = 0, A₄ = 5.9595 × 10⁻⁵, A₆ = −8.2829 × 10⁻⁷,A₈ = 1.1533 × 10⁻⁸, A₁₀ = −4.9667 × 10⁻¹¹ (Twenty-first Order) k = 0, A₄= −7.9720 × 10⁻⁵, A₆ = −2.0100 × 10⁻⁷, A₈ = 2.0113 × 10⁻⁹, A₁₀ = 7.2706× 10⁻¹⁴ (Twenty-second Order) k = 0, A₄ = −4.3301 × 10⁻⁵, A₆ = −1.5941 ×10⁻⁷, A₈ = 3.0911 × 10⁻⁹, A₁₀ = −7.0460 × 10⁻¹² (Focal State Data)Minimum Object Distance Infinity (object distance 158.000 mm) D(14)1.489 2.539 D(16) 6.373 5.323 f (focal length of optical 16.49 16.05system overall) FNO (F number) 2.88 2.91 ω (half angle of view) 53.5053.14 f1 (focal length of first 10.68 10.68 lens group G₃₁) f2 (focallength of second −61.48 −61.48 lens group G₃₂) f3 (focal length of third−126154 −126154 lens group G₃₃) BF (back focus) 23.838 23.838

(Values Related to Conditional Expression (1))

f3/f=−7652.47

(Values Related to Conditional Expression (2))

f1/f=0.65

(Values Related to Conditional Expression (3))

βinf (paraxial magnification of second lens group G₃₂ at focus state forobject at infinity)=1.59β mod (paraxial magnification of second lens group G₃₂ at focus statefor minimum object distance)=1.57βinf/β mod=1.01

(Values Related to Conditional Expression (4))

R1 (radius of curvature of surface at air interface on object side ofnegative lens L₃₃)=−400.000R2 (radius of curvature of surface at air interface on image side ofnegative lens L₃₃₃)=34.153

(R1/R2)/(R1−R2)=0.84 (Values Related to Conditional Expression (5))

L1s (axial distance from surface farthest on object side of first lensgroup G₃, to aperture stop STP)=43.025L (axial distance from apex of lens surface farthest on object side offirst lens group G₃₁ to imaging plane IMG (overall length of opticalsystem))=92.000

L1s/L=0.47 (Values Related to Conditional Expression (6))

f2/f=−3.73

(Values Related to Conditional Expression (7))

νdn (Abbe number with respect to d-line of negative lens L₃₃₃)=37.22

(Values Related to Conditional Expression (8)

R21 (radius of curvature of surface farthest on object side of negativelens L₃₂₁)=168.747R22 (radius of curvature of surface farthest on image side of negativelens L₃₂₁)=35.344

(R21+R22)/(R21−R22)=1.53

FIG. 6 is a diagram of various types of aberration occurring in theinner focusing lens according to the third embodiment. In the diagram,for curves depicting spherical aberration, the vertical axis representsthe F number (Fno), solid lines depict wavelength characteristicscorresponding to d-line (λ=587.56 nm), dotted lines depict wavelengthcharacteristics corresponding to g-line (λ=435.84 nm), and dashed linesdepict wavelength characteristics corresponding to C-line (λ=656.28 nm).For curves depicting astigmatism, the vertical axis represents themaximum image height (Y), S represents characteristics of the sagittalplane and M represents characteristics of the meridional plane. Forcurves depicting distortion, the vertical axis represents the maximumimage height (Y) and wavelength characteristics corresponding to d-lineare depicted.

FIG. 7 is a diagram depicting, along the optical axis, a configurationof the inner focusing lens according to a fourth embodiment. FIG. 7depicts a focus state for an object at infinity. The inner focusing lenshas sequentially from the object side, a first lens group G₄₁ having apositive refractive power, a second lens group G₄₂ having a negativerefractive power, and a third lens group G₄₃ having a negativerefractive power. The cover glass CG is disposed between the third lensgroup G₄₃ and the imaging plane IMG.

The first lens group G₄₁ has sequentially from the object side, anegative meniscus lens L₄₁₁, a negative meniscus lens L₄₁₂, a negativelens L₄₁₃, a positive lens L₄₁₄, a negative lens L₄₁₅, a positive lensL₄₁₅, a positive lens L₄₁₇, and the aperture stop STP setting apredetermined diameter. Both surfaces of the negative lens L₄₁₃ areaspheric. The negative lens L₄₁₅ and the positive lens L₄₁₆ arecemented. Both surfaces of the positive lens L₄₁₇ are aspheric.

The second lens group G₄₂ is configured by a negative lens L₄₂₁. Bothsurfaces of the negative lens L₄₂₁ are aspheric.

The third lens group G₄₃ has sequentially from the object side, a frontlens subgroup G_(43F) having a positive refractive power and a rear lenssubgroup G_(43R) having a negative refractive power. An axial air gapthat is widest in the third lens group G₄ is formed between the frontlens subgroup G_(43F) and the rear lens subgroup G_(43R).

The front lens subgroup G₄₃F has sequentially from the object side, apositive lens L₄₃₁ and a positive lens L₄₃₂ The rear lens subgroupG_(43F) is configured by a negative lens L₄₃₃. The negative lens L₄₃₃ isformed of a single glass material. Both surfaces of the negative lensL₄₃₃ are aspheric.

In the inner focusing lens, the first lens group G₄₁ and the third lensgroup G₄₃ are fixed, while the second lens group G₄₂ is moved along theoptical axis from the object side to the imaging plane IMG side suchthat the interval between the first lens group G₄₁ and the second lensgroup G₄₂ is increased and the interval between the second lens groupG₄₂ and the third lens group G₄₃ is decreased, whereby focusing isperformed from a focus state for an object at infinity to a focus statefor the minimum object distance.

Here, various values related to the inner focusing lens according to thefourth embodiment are given.

(Lens Data) r₁ = 28.832 d₁ = 2.0000 nd₁ = 1.5935 νd₁ = 67.00 r₂ = 15.623d₂ = 6.8857 r₃ = 29.397 d₃ = 1.5000 nd₂ = 1.4970 νd₂ = 81.61 r₄ = 13.122d₄ = 5.8831 r₅ = 52.568 (aspheric surface) d₅ = 1.3000 nd₃ = 1.5920 νd₃= 67.02 r₆ = 12.910 (aspheric surface) d₆ = 3.0680 r₇ = 30.333 d₇ =2.2648 nd₄ = 1.8810 νd₄ = 40.14 r₈ = 141.737 d₈ = 12.0987 r₉ = 24.676 d₉= 1.0000 nd₅ = 1.8810 νd₅ = 40.14 r₁₀ = 13.601 d₁₀ = 5.2921 nd₆ = 1.4875νd₆ = 70.44 r₁₁ = −35.502 d₁₁ = 3.7248 r₁₂ = 30.850 (aspheric surface)d₁₂ = 4.3865 nd₇ = 1.4971 νd₇ = 81.56 r₁₃ = −18.338 (aspheric surface)d₁₃ = 1.0000 r₁₄ = ∞ (aperture stop) d₁₄ = D(14) (variable) r₁₅ = 45.781(aspheric surface) d₁₅ = 0.8000 nd₈ = 1.7290 νd₈ = 54.04 r₁₆ = 19.193(aspheric surface) d₁₆ = D(16) (variable) r₁₇ = −68.737 d₁₇ = 2.5350 nd₉= 1.4970 νd₉ = 81.61 r₁₈ = −25.555 d₁₈ = 0.1000 r₁₉ = −329.577 d₁₉ =4.0360 nd₁₀ = 1.4970 νd₁₀ = 81.61 r₂₀ = −23.042 d₂₀ = 0.3290 r₂₁ =−400.000 (aspheric surface) d₂₁ = 1.2000 nd₁₁ = 1.8820 νd₁₁ = 37.22 r₂₂= 26.785 (aspheric surface) d₂₂ = 25.606 r₂₃ = ∞ d₂₃ = 2.5000 nd₁₂ =1.5168 νd₁₂ = 64.20 r₂₄ = ∞ d₂₄ = 1.0000 r₂₅ = ∞ (imaging plane)Constant of the Cone (k) and Aspheric Coefficients (A₄, A₆, A₈, A₁₀)(Fifth Order) k = 0, A₄ = −2.6625 × 10⁻⁵, A₆ = 3.0031 × 10⁻⁷, A₈ =−1.7989 × 10⁻⁹, A₁₀ = 5.7847 × 10⁻¹² (Sixth Order) k = 0, A₄ = −6.8334 ×10⁻⁵, A₆ = 8.4364 × 10⁻⁹, A₈ = −1.2228 × 10⁻⁹, A₁₀ = −9.8374 × 10⁻¹²(Twelfth Order) k = 0, A₄ = −2.2269 × 10⁻⁵, A₆ = 1.1253 × 10⁻⁹, A₈ =−1.0562 × 10⁻⁹, A₁₀ = −3.1969 × 10⁻¹² (Thirteenth Order) k = 0, A₄ =5.1791 × 10⁻⁵, A₆ = −5.2286 × 10⁻⁷, A₈ = 4.0083 × 10⁻⁹, A₁₀ = −2.3692 ×10⁻¹¹ (Fifteenth Order) k = 0, A₄ = 2.0348 × 10⁻⁵, A₆ = −1.1141 × 10⁻⁶,A₈ = 1.4175 × 10⁻⁸, A₁₀ = −5.7786 × 10⁻¹¹ (Sixteenth Order) k = 0, A₄ =2.1580 × 10⁻⁵, A₆ = −8.9505 × 10⁻⁷, A₈ = 1.2780 × 10⁻⁸, A₁₀ = −5.7413 ×10⁻¹¹ (Twenty-first Order) k = 0, A₄ = −2.4151 × 10⁻⁵, A₆ = −1.3394 ×10⁻⁷, A₈ = 2.2182 × 10⁻⁹, A₁₀ = −7.5852 × 10⁻¹² (Twenty-second Order) k= 0, A₄ = −3.5206 × 10⁻⁶, A₆ = −1.2925 × 10⁻⁷, A₈ = 2.2655 × 10⁻⁹, A₁₀ =−8.5817 × 10⁻¹² (Focal State Data) Minimum Object Distance Infinity(object distance 158.000 mm) D(14) 0.4719 1.3177 D(16) 5.8194 4.9736 f(focal length of optical 18.54 17.89 system overall) FNO (F number) 2.882.93 ω (half angle of view) 50.29 49.47 f1 (focal length of first 10.2810.28 lens group G₄₁) f2 (focal length of second −45.92 −45.92 lensgroup G₄₂) f3 (focal length of third −722.75 −722.75 lens group G₄₃) BF(back focus) 29.106 29.106

(Values Related to Conditional Expression (1))

f3/f=−38.97

(Values Related to Conditional Expression (2))

f1/f=0.55

(Values Related to Conditional Expression (3))

βinf (paraxial magnification of second lens group G₄₂ at focus state forobject at infinity)=1.83β mod (paraxial magnification of second lens group G₄₂ at focus statefor minimum object distance)=1.81βinf/β mod=1.01

(Values Related to Conditional Expression (4))

R1 (radius of curvature of surface at air interface on object side ofnegative lens L₄₃)=−400.000R2 (radius of curvature of surface at air interface on image side ofnegative lens L₄₃₃)=26.785

(R1/R2)/(R1−R2)=0.87 (Values Related to Conditional Expression (5))

L1s (axial distance from surface farthest on object side of first lensgroup G₄, to aperture stop STP)=50.4037L (axial distance from apex of lens surface farthest on object side offirst lens group G₄₁ to imaging plane IMG (overall length of opticalsystem))=94.8010

L1s/L=0.53 (Values Related to Conditional Expression (6))

f2/f=−2.48

(Values Related to Conditional Expression (7))

νdn (Abbe number with respect to d-line of negative lens L₄₃₃)=37.22

(Values Related to Conditional Expression (8))

R21 (radius of curvature of surface farthest on object side of negativelens L₄₂₁)=45.781R22 (radius of curvature of surface farthest on image side of negativelens L₄₂₁)=19.193

(R21+R22)/(R21−R22)=2.44

FIG. 8 is a diagram of various types of aberration occurring in theinner focusing lens according to the fourth embodiment. In the diagram,for curves depicting spherical aberration, the vertical axis representsthe F number (Fno), solid lines depict wavelength characteristicscorresponding to d-line (λ=587.56 nm), dotted lines depict wavelengthcharacteristics corresponding to g-line (λ=435.84 nm), and dashed linesdepict wavelength characteristics corresponding to C-line (λ=656.28 nm).For curves depicting astigmatism, the vertical axis represents themaximum image height (Y), S represents characteristics of the sagittalplane and M represents characteristics of the meridional plane. Forcurves depicting distortion, vertical axis represents the maximum imageheight (Y) and wavelength characteristics corresponding to d-line aredepicted.

Among the values for each of the embodiments, r₁, r₂, . . . indicate theradius of curvature of lens surfaces, diaphragm surface, etc.; d₁, d₂, .. . indicate the thickness of the lenses, the diaphragm, etc. or theinterval between the surfaces thereof; nd₁, nd₂, . . . indicate therefraction index of the lenses with respect to the d-line (λ=587.56 nm);and νd₁, νd₂, . . . indicate the Abbe number for the d-line (λ=587.56nm) of the lenses. Further, back focus (BF) represents the distance fromthe last surface of the optical system to the paraxial imaging plane.The overall length of the optical system is the distance from thesurface farthest on the object side to the last lens surface plus theBF. Lengths are indicated in units of “mm”; and angles are indicated in“degrees”.

Each aspheric surface shape above is expressed by the equation below;where, Z is the depth of the aspheric surface, c(1/r) is curvature; h isthe height from the optical axis; k is the constant of the cone; A₄, A₆,A₆, A₁₀ are respectively fourth order, sixth order, eighth order, andtenth order aspheric coefficients; and the travel direction of light isassumed to be positive.

$Z = {\frac{{ch}^{2}}{1 + \sqrt{1 - {( {1 + k} )c^{2}h^{2}}}} + {A_{4}h^{4}} + {A_{6}h^{6}} + {A_{8}h^{8}} + {A_{10}h^{10}}}$

In the embodiments, an example of an inner focusing lens having focallengths accommodating wide angles of view by a 35 mm film cameraconversion has been given. The inner focusing lens of the embodimentsfacilitates reductions in the size and weight of the focusing group andtherefore, can favorably perform high-speed autofocus processing,essential for video filming. In particular, by satisfying theconditional expressions above, an inner focusing lens that is optimalfor capturing video, is compact, and has high imaging performance atwide angles can be realized.

According to the embodiments, a compact inner focusing lens having highimaging performance, a wide angle focal length, and a compact,light-weight focusing group can be realized.

According to the embodiments, an inner focusing lens can be realizedthat is even more compact and has high imaging performance.

According to the embodiments, field curvature consequent to focusing canbe suppressed, enabling improved imaging performance.

According to the embodiments, the diameter of the lens near the imagingplane can be reduced, and on-axis aberration and off-axis aberration(particularly, distortion) can be favorably corrected.

According to the embodiments, the diameter of the third lens group (lensfarthest on the image side) can be reduced and off-axis coma aberrationcan be favorably corrected.

According to the embodiments, front lens and rear lens diameters can bereduced while maintaining imaging performance, enabling furtherreductions in the size of the optical system.

According to the embodiments, reduction the size and weight of thesecond lens group, which is the focusing group, are facilitated,enabling an inner focusing lens suitable for capturing video to beprovided.

According to the embodiments, the overall length of the optical systemcan be reduced and imaging performance can be improved.

According to the embodiments, the simple lens element having a negativerefractive power and included in the third lens group can be reduced insize and weight, and chromatic difference of magnification can befavorably corrected.

According to the embodiments, an effect is achieved in that a compactinner focusing lens having high imaging performance, a wide angle focallength, and a compact, light-weight focusing group can be provided.According to the embodiments, a compact inner focusing lens also optimalfor capturing video can be provided.

As described, the inner focusing lens according to the present inventionis useful for compact imaging apparatuses such as still image camerasand video cameras, and is particularly suitable for imaging apparatusesused for capturing moving images.

Although the invention has been described with respect to a specificembodiment for a complete and clear disclosure, the appended claims arenot to be thus limited but are to be construed as embodying allmodifications and alternative constructions that may occur to oneskilled in the art which fairly fall within the basic teaching hereinset forth.

What is claimed is:
 1. An inner focusing lens comprising sequentiallyfrom an object side thereof: a first lens group having a positiverefractive power; a second lens group having a negative refractivepower; and a third lens group having a negative refractive power,wherein the first lens group includes farthest on an object sidethereof, at least one negative meniscus lens, focusing is performed froma focus state at infinity to a focus state for a minimum object distanceby moving the second lens group along an optical axis, from the objectside to an image side such that an interval between the first lens groupand the second lens group increases and an interval between the secondlens group and the third lens group decreases, while the first lensgroup and the third lens group remain fixed, and the inner focusing lenssatisfies a conditional expression (1) f3/f≦−29.1, where f3 is a focallength of the third lens group at the focus state at infinity and f isan overall optical-system focal length at the focus state at infinity.2. The inner focusing lens according to claim 1, wherein the innerfocusing lens satisfies a conditional expression (2) 0.28≦f1/f≦1.30,where f1 is a focal length of the first lens group at the focus state atinfinity.
 3. The inner focusing lens according to claim 1, wherein theinner focusing lens satisfies a conditional expression (3) 0.50≦βinf/βmod≦2.02, where βinf is paraxial magnification of the second lens groupat the focus state at infinity and β mod is the paraxial magnificationof the second lens group at the focus state for the minimum objectdistance.
 4. The inner focusing lens according to claim 1, wherein thethird lens group includes sequentially from an object side thereof, afront lens subgroup having a positive refractive power and a rear lenssubgroup having a negative refractive power, and an axial air gap thatis widest in the third lens group is formed between the front lenssubgroup and the rear lens subgroup.
 5. The inner focusing lensaccording to claim 1, wherein the third lens group includes farthest onan image side thereof, a simple lens element having a negativerefractive power, and the inner focusing lens satisfies a conditionalexpression (4) 0≦(R1+R2)/(R1−R2), where R1 is radius of curvature of asurface of the simple lens element, at an air interface on an objectside of the simple lens element and R2 is radius of curvature of asurface of the simple lens element, at the air interface on an imageside of the simple lens element.
 6. The inner focusing lens according toclaim 1 and further comprising an aperture stop that sets apredetermined diameter, wherein the inner focusing lens satisfies aconditional expression (5) 0.24≦L1s/L≦0.95, where L1s is an axialdistance from a surface farthest on the object side of the first lensgroup to the aperture stop and L is the overall optical-system focallength, which is an axial distance from an apex of a lens surfacefarthest on the object side in the optical system to an imaging plane.7. The inner focusing lens according to claim 1, wherein the second lensgroup is configured by a simple lens element having a negativerefractive power.
 8. The inner focusing lens according to claim 1,wherein the inner focusing lens satisfies a conditional expression (6)−7.46≦f2/f≦−2.11, where f2 is a focal length of the second lens group atthe focus state at infinity.
 9. The inner focusing lens according toclaim 1, wherein the third lens group includes a simple lens elementhaving a negative refractive power, the simple lens element is formed ofa single glass material, and the inner focusing lens satisfies aconditional expression (7) 30≦νdn, where νdn is an Abbe number withrespect to d-line of the simple lens element.